To my own mother, it was an article of faith to show no favoritism. If two of us asked her whose drawing she liked best, the answer was predetermined: I like them just the same. When I tried to trick my mother by saying I had done both drawings myself, she saw right through me; she understood that children are constantly trying to elicit evidence of who is ahead and who is behind. And to the end of her life, if someone tried to draw my mother out in public praise, by saying, with reference to some particular milestone or achievement, oh, you must be so proud of your child, she would respond, firmly, yes, I’m proud of all my children.

Her parents, back in the 1930s, had no concerns about treating children equally; the boy was the boy was the boy; the girls were the smart one and the pretty one.

Dr. Barbara Howard, a developmental behavioral pediatrician who is the president of Total Child Health and an assistant professor of pediatrics at Johns Hopkins University School of Medicine, often sees behavioral problems that stem from a child’s sense of not being the preferred one. “It’s impossible not to have favorites, and we do know that the perception of favoritism is one of the biggest factors in sibling rivalry,” she said.

“Often the child is trying to get the attention of the parent who is rejecting them — the more you push a kid away, the more he will come at you,” she said. “So if you see a kid coming at a parent, being aggressive or being clingy or needy or overly attention-seeking, often the parent doesn’t like the kid that much, or the kid perceives it.” She may ask the parent what that child’s behavior evokes; which other family member does it make you think of; what possible future does it make you imagine? Often, she says, the parent is aware of feeling strained toward that child, and feels terribly guilty about it; finding ways to enjoy spending time together can help them both.

Years ago I read a novel — someone please tell me what it was — in which a mother secretly and privately assured each of her children, don’t tell the others, but you have always been my favorite. I liked that system, and, as a mother, I think I could do it with perfect sincerity — one on one with each of my three children, I think I could say it and it would be true.

Ellen Weber Libby, a clinical psychologist and author of “The Favorite Child,” said some families have a shifting favoritism, where different children hold the advantage from day to day or week to week. That kind of rotation, she said, yields a healthy, normal competitiveness. Ask the children, she says, and they will tell you. “The people who don’t know are usually the parents, who live in denial because there’s a myth that to have a favorite child is bad.”

The danger comes when the favoritism is steady and persistent and becomes a lasting part of the family dynamic.

Evolutionary psychologists think of parental investment in their offspring as the division of a finite pool of resources, rather than, perhaps, an infinity of love. “I would argue that parents do sometimes have favorites and do invest unequally,” said Catherine Salmon, an associate professor of psychology at the University of Redlands in California, who studies relationships and is a co-author of “The Secret Power of Middle Children.” Birth order can matter here, she said, with middle children perhaps less likely to be favorites, compared with first children, who monopolize their parents, for that first period, and last children, who represent a final chance to invest.

Dr. Salmon pointed out that the effects of parental favoritism may be much sharper in families where there isn’t enough to go around in the first place, so the inequities may be particularly harsh. On the other hand, Dr. Libby said, in a prosperous family, the favorite child may grow up entitled, immune from the rules that apply to the other children.

“I think you can let people off the hook from feeling guilty about having a favorite — put it right out there and say of course you have a favorite, people have favorites, it’s what you do with it that matters,” said Dr. Howard. “You’ve got to find something you appreciate about each kid and build on that.” With children whose behavior is problematic, she may suggest developing new rituals, like an early-morning cuddle before the day gets going.

“Parents don’t appreciate the difference between love and favoritism,” said Dr. Libby. “I think it’s hard for parents to say, I love my children the same and from time to time there is a child I do favor. I favor a child because at that moment that child makes me feel more successful as a parent.”

So yes, there may be real inequities — but what may matter more is the perception of favoritism, and what everyone involved does with it, both in terms of behavior, and in terms of memory and emotion. We all carry with us into adulthood a sense of where we stood, how we were perceived and how we were treated.

On a good day, the idea of the favorite child can be a bit of a running joke, which serves as a reminder to parents to play fair, and as a reminder to children that while love is infinite, parental approval and esteem need to be earned, and are worth competing for, within reason.

When Dr. Libby had to put together her first PowerPoint presentation, she said, she found herself feeling overwhelmed. She texted her children: “Whoever gets back to me first is my favorite child for today.”

“Within a nanosecond my daughter, who never has time to call me, was on the phone, and my son said, damn, when your phone was busy I knew my sister was on it!”